DENVER (AP) – New General Motors CEO Mary Barra has been cooperative with members of Congress investigating why the company did not act sooner to address a potentially deadly defect in some of its small cars, U.S. Rep. Diana DeGette said.

DeGette, a Democrat from Colorado who is the ranking member of the House committee investigating GM’s recall of 1.6 million cars, said Friday that she appreciates Barra’s responsiveness.

But questions remain about why GM declined for a decade to alert car owners about faulty ignition switches that can causes a car’s engine to stall unexpectedly, DeGette said. GM last month recalled Chevrolet Cobalts and other compact cars, including the Saturn Ion, Pontiac G5 and Chevrolet HHR, to have the switches repaired.

The defect has been linked to at least 12 deaths and 31 crashes, DeGette said.

Later Friday, it was announced that General Motors was boosting by 971,000 the number of small cars being recalled worldwide for a defective ignition switch, saying cars from the model years 2008-2011 may have received the part as a replacement.

During congressional hearings that start Tuesday, DeGette said she will also press the National Highway Transportation Safety Administration about why they never ordered GM to recall the cars. An analysis by The Associated Press showed the agency had evidence of the problematic switches since at least 2005.

“We need to find out when GM knew, why they didn’t inform the public, and what they intend to do to make sure all of the cars that are on the road are safe to drive,” DeGette said.

GM has said the cars are safe, if drivers remove everything from their key rings. It will take until October to fix all the recalled cars.

DeGette also said she has questions about why GM told the government that drivers would get loaner cars while their vehicles were being fixed, but the safety recall makes no mention of that possibility.

GM fixed the switches on later models by changing the design but did not give the new switches a different part number, so it is impossible to know which ones are faulty, DeGette said.

Danny Katz, state director of the consumer advocate Colorado Public Interest Research Group, advised owners of recalled cars to keep them parked for now.